lib/, src/: Use streq() instead of its pattern

lib/, src/: Use streq() instead of its pattern

Except for the added (and sorted) includes, the removal of redundant
parentheses, a few cases that have been refactored for readability, and
a couple of non-string cases that I've left out of the change, this
patch can be approximated with the following semantic patch:

	$ cat ~/tmp/spatch/streq.sp
	@@
	expression s;
	@@

	- '\0' == *s
	+ streq(s, "")

	@@
	expression s;
	@@

	- '\0' == s[0]
	+ streq(s, "")

	@@
	expression s;
	@@

	- *s == '\0'
	+ streq(s, "")

	@@
	expression s;
	@@

	- s[0] == '\0'
	+ streq(s, "")

	$ find contrib/ lib* src/ -type f \
	| xargs spatch --in-place --sp-file ~/tmp/spatch/streq.sp;

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