Rewrite qual_desc using bit sets

Rewrite qual_desc using bit sets

As a side effect, this also fixes support of negated sets
of descriptors.

* defs.h (struct number_set): New forward declaration.
(read_set, write_set): New variable prototypes.
(is_number_in_set, qualify_read, qualify_write): New function
prototypes.
(QUAL_READ, QUAL_WRITE): Change to values greater than 0xff.
* qualify.c: New file.
* Makefile.am (strace_SOURCES): Add it.
* syscall.c (qual_desc): Remove.
(qual_options): Replace qual_desc with NULL.
(qualify): Use qualify_read and qualify_write.
(dumpio): Use is_number_in_set.
* tests/options-syntax.test: Check invalid sets of descriptors.
* tests/readv.test: Check dumping of negated sets of descriptors.