emacs: show: make return value of notmuch-show-get-prop explicit

This makes the fact the notmuch-show-get-prop returns nil if the major
mode is neither show not tree explicit.
emacs: Clarify that notmuch-poll-script is deprecated

notmuch-poll-script has long since been deprecated in favor of
post-new hooks, but this wasn't obvious from the documentation.
Update the documentation to make this clear.  Since
notmuch-poll-script could, to some extend, be used to control the path
of the notmuch binary and that use is now clearly discouraged, promote
notmuch-command to a real defcustom instead of just a variable.
util: Make string-util.h C++-compatible
config: read user.name from $NAME if set

Try to read the config parameter user.name from $NAME before taking the
user name from /etc/passwd.
config: read database.path from $MAILDIR if set

Try to read the config parameter database.path from $MAILDIR before
falling back to $HOME/mail