Revert r269523:

Providing a higher EXT2_LINK_MAX limit is a bad idea for ext2/3.

Discussed with:	bde
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.