auxv: partially revert r366207, cast buflen to unsigned int as needed

The warning generated pre-r366207 is actually a sign comparison warning:

error: comparison of integers of different signs: 'unsigned long' and 'int'

if (strlcpy(buf, execpath, buflen) >= buflen)

Revert parts that affected other lines and just cast this to unsigned int.

The buflen < 0 -> EINVAL has been kept despite no longer serving any
purposes w.r.t. sign-extension because I do believe it's the right thing to
do: "The provided buffer was not the right size for the requested item."

The original warning is confirmed to still be gone with an:
env WARNS=6 make WITHOUT_TESTS=yes.

Reviewed by: asomers, kib
X-MFC-With: r366207
Differential Revision: https://reviews.freebsd.org/D26631