Fix several new & old style issues.

Pointed by:	ae, pluknet, zont
MFC with:	r247036
Add a -D flag that causes duplicate entries in an mtree manifest to be
treated as warnings rather than errors.

Reviewed by:	marcel
Sponsored by:	DARPA, AFRL
Fix the -N option in manifest mode by using pwcache(3).  This also
speeds up image creation appreciably.

Reviewed by:	marcel
Sponsored by:	DARPA, AFRL
Allow '.' components in manifest paths.  They are always the first
component of mtree -C and install -M output and are easily skipped.

Reviewed by:	marcel
Sponsored by:	DARPA, AFRL
ip_savecontrol() style fixes. No functional changes.
- fix indentation
- put the operator at the end of the line for long statements
- remove spaces between the type and the variable in a cast
- remove excessive parentheses

Tested by:	md5