Import bmake-20230510

Relevant/interesting changes:

o parse.c: don't print null filename in stack traces
o for.c: skip syntactically wrong .for loops
o var.c: allow for :gmtime=${mtime}
add :mtime[=timestamp] where timestamp is used if stat(2)
fails, if :mtime=error stat(2) failure causes error.
o make.1: fix documentation of .PREFIX to match reality and POSIX
o unit-tests: improved var-scope-local