Use GitHub Actions for builds

[Michael Mattioli]

I pushed a commit to my fork which replaces Travis and AppVeyor with GitHub
Actions for builds. You can see the commit here:
https://github.com/mmattioli/krb5/commit/3ecd36d4b701f2e6f01f6148c51d246ad001880f

Why? Was curious to see if it would work and how well it would work.
Wondering if there's an appetite to help me work through some bugs and
replace them or not. Just curious. Some notes:

- I had to remove the installation of slapd on the linux builds because the
package it was trying to install was asking for user input which isn't
possible in an automated environment. Despite removing it from
installation, there are no errors in the three linux build variants.
- The Windows build is failing at what seems to be a syntax error
somewhere? The output from the log where it seems to be failing is from
this command: perl -w .\../util/def-check.pl private-and-public-decls
.\../lib/krb5_32.def

2019-12-02T03:50:45.1330582Z Microsoft (R) Program Maintenance Utility
Version 14.23.28106.4
2019-12-02T03:50:45.1331518Z Copyright (C) Microsoft Corporation.  All
rights reserved.
2019-12-02T03:50:45.1331878Z
2019-12-02T03:50:45.1383429Z copy/b nul:+ win-mac.h autoconf.h
2019-12-02T03:50:45.1564127Z nul
2019-12-02T03:50:45.1570480Z win-mac.h
2019-12-02T03:50:45.1890748Z         1 file(s) copied.
2019-12-02T03:50:45.1907748Z copy/b nul:+ osconf.hin osconf.h
2019-12-02T03:50:45.2068478Z nul
2019-12-02T03:50:45.2072899Z osconf.hin
2019-12-02T03:50:45.2079706Z         1 file(s) copied.
2019-12-02T03:50:45.2095011Z copy /y
krb5\krb5.h+k5-int.h+krb5\clpreauth_plugin.h private-and-public-decls
2019-12-02T03:50:45.2251785Z krb5\krb5.h
2019-12-02T03:50:45.2265156Z k5-int.h
2019-12-02T03:50:45.2268641Z krb5\clpreauth_plugin.h
2019-12-02T03:50:45.2273313Z         1 file(s) copied.
2019-12-02T03:50:45.2287382Z perl -w .\../util/def-check.pl
private-and-public-decls .\../lib/krb5_32.def
2019-12-02T03:50:46.4356046Z Invalid function name: '
2019-12-02T03:50:46.4356385Z
2019-12-02T03:50:46.4356597Z
2019-12-02T03:50:46.4356745Z  as in DCE, or for users */
2019-12-02T03:50:46.4356929Z  */
2019-12-02T03:50:46.4357105Z  NT_WELLKNOWN principals */
2019-12-02T03:50:46.4358065Z
2019-12-02T03:50:46.4359329Z
2019-12-02T03:50:46.4359706Z  typedef const krb5_principal_data
*krb5_const_principal
2019-12-02T03:50:46.4359905Z
2019-12-02T03:50:46.4360478Z ' at .\../util/def-check.pl line 175, <GEN0>
line 258.
2019-12-02T03:50:46.4598010Z NMAKE : fatal error U1077: '"C:\Program
Files\Git\usr\bin\perl.EXE"' : return code '0xff'
2019-12-02T03:50:46.4598317Z Stop.
2019-12-02T03:50:46.4632550Z NMAKE : fatal error U1077: 'for' : return code
'0x1'
2019-12-02T03:50:46.4632877Z Stop.


- You can checkout the 4x build runs here:
https://github.com/mmattioli/krb5/commit/1c6122ea55f7ec5f681a8bcc7e825bd4e20b5207/checks?check_suite_id=337012953

Mike