Replaced boilerplate at beginning of each nmh program with new

Replaced boilerplate at beginning of each nmh program with new
nmh_init() function.  It sets up an atexit() function and signal
handlers so that all temporary files are removed when the program
terminates, however that happens.  It relies on a call in m_mktemp()
to register each temporary file for removal.  See new "nmh temporary
files" section in README.developers and comments in m_mktemp.c.