lib/, src/: Rename some local variables

lib/, src/: Rename some local variables

'endptr' is appropriate internally in strtol(3) because it's a pointer
to 'end', and 'end' itself is a pointer to one-after-the-last character
of the numeric string.  In other words,

	endptr == &end

However, naming the pointer whose address we pass to strtol(3)'s
'endptr' feels wrong, and causes me trouble while parsing the code; I
need to double check the number of dereferences, because something feels
wrong in my head.

Signed-off-by: Alejandro Colomar <alx@kernel.org>