Ignored signals, such as SIGPIPE in Mutt, are inherited by child
processes.  This can cause incorrect behavior in filter-like programs
that expect SIGPIPE to be the default value of "Term".

A couple other places in Mutt already reset SIGTERM, SIGTSTP, and
SIGCONT, with a comment mentioning it really wasn't necessary.  Since
I don't much like messing with signals, I'll leave those resets in.

Extract those plus SIGPIPE into a new function,
mutt_reset_child_signals() and add that before each exec call.

Thanks to Vincent Lefèvre for reporting the issue and for the initial
patch, which this commit is based upon.