Improvements to the diagnostics for "unimplemented" POSIX functions.

Improvements to the diagnostics for "unimplemented" POSIX functions.

Suggested by Aristotle Pagaltzis as part of a larger refactoring.
This regularises the text, changing '--use' and ': use' to ', use' to be
consistent, provides a message for POSIX::srand(), and adds ' is' to the
message for POSIX::bsearch().

Most of the diagnostics have been unchanged since perl 5.000. For some,
IO::Handle replaced FileHandle in perl5.003_20 (28757baaaeaa3801).
div and ldiv's messages were improved to also mention % in 2003 by commit
7a6ca5fd18d88091.