MFC r269550:

MFC r269550:
r269550 (by peter):

Check gethostname(2) return code - but even if it succeeds it may not
null terminate.

Temporarily use "From: $user@$hostname" rather than "From: $user".
The latter exposes incompatible behavior if using dma(8).  sendmail(8)
(and other alternatives) canonify either form on submission (even
if masquerading), but dma will leak a non-compliant address to
the internet.

git-svn-id: svn://svn.freebsd.org/base/stable/10@321244 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f