R50c is a security fix release:

• Know more rare signals when generating sys_signame[] replacement
• OpenBSD sync (mostly RCSID only)
• Document HISTSIZE limit; found by luigi_345 on IRC
• Fix link to Debian .mkshrc
• Cease exporting $RANDOM (Debian #760857)
• Fix C99 compatibility
• Work around klibc bug causing a coredump (Debian #763842)
• Use issetugid(2) as additional check if we are FPRIVILEGED
• SECURITY: do not permit += from environment
• Fix more field splitting bugs reported by Stephane Chazelas and mikeserv; document current status wrt. ambiguous ones as testcases too