mail: don't complain about a multi-word email.method
I want to be able to set email.method to "ssh relay /usr/sbin/sendmail"
without needing an extra trivial shell script.
This works fine since we pass the full command to a shell, except for
validateconfig trying to find it in $PATH.
Differential Revision: https://phab.mercurial-scm.org/D7542