MFC r268001:
Make fdunshare accept only td parameter.

Proc had to match the thread anyway and 2 parameters were inconsistent
with the rest.
MFH: r264781, r267669, r267670

Simplify reading pw.conf(5) by using getline(3)

Removed compatibility with pre FreeBSD 2.2 pw_mkdb command [1]
Fix some broken indentattion [1]

Fix changing the username [2]

PR:		189172 [1], 189173 [2]
Submitted by:	fullermd@over-yonder.net [1][2]
MFC r268002:

Call fdcloseexec right after fdunshare.

No functional changes.