xutils: Make len in strtrim_{right,left}() size_t

strlen(3) returns size_t, so use a variable of this type to store the
length.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
xutils: Don't use register keyword

Today's compilers are better at determining the use of registers by
themselves.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>