Introduce a uiomove_frombuf helper routine that handles computing and

 Introduce a uiomove_frombuf helper routine that handles computing and
 validating the offset within a given memory buffer before handing the
 real work off to uiomove(9).

 Use uiomove_frombuf in procfs to correct several issues with
 integer arithmetic that could result in underflows/overflows.  As a
 side-effect, the code is significantly simplified.

 Add additional sanity checks when computing a memory allocation size
 in pfs_read.

 Reported by:    Joost Pol <joost@pine.nl>  (integer underflows/overflows)
 Originated from: FreeBSD