pycompat: adding Linux detection and fixing Mac

Python 3 recommends detecting OSs with the prefix of the platform, but we were
comparing the full string for macOS. We also didn't have Linux detection, which
is convenient for extensions to use (rather than have some OSs detected by hg
and some by the extension).

Reference:
https://docs.python.org/3/library/sys.html#sys.platform

Differential Revision: https://phab.mercurial-scm.org/D5227

mercurial/pycompat.py

@@ -403,7 +403,8 else:
 
 isjython = sysplatform.startswith(b'java')
 
-isdarwin = sysplatform == b'darwin'
+isdarwin = sysplatform.startswith(b'darwin')
+islinux = sysplatform.startswith(b'linux')
 isposix = osname == b'posix'
 iswindows = osname == b'nt'