internal: Add STRLIM macro for checking string length using strnlen()

internal: Add STRLIM macro for checking string length using strnlen()

As a microoprimization when checking whether length of a string fits
into a limit we don't necessarily need to calculate the full length but
can use strnlen to check only LIMIT+1 chars. Add a macro which will
simplify the expressions.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>