secure_getenv: Port to many more platforms. * m4/secure_getenv.m4 (gl_PREREQ_SECURE_GETENV): Also check for get*id functions. * lib/secure_getenv.c (secure_getenv): Add alternate implementations for non-BSD Unix platforms and for native Windows. * doc/glibc-functions/secure_getenv.texi: Remove known issue. Prompted by a request from Nikos Mavrogiannopoulos.