lib/utils.c: lazy initialize user_hz and psched_hz

Rather than initializing user_hz and psched_hz when libnl is loaded,
defer initialization of these variables to the first time they are used.
This has several advantages:

1) Avoids an unnecessary permission denied error on /proc/net/psched,
which can occur on systems where /proc/net isn't readable due to
security policy.
2) Allows program code to initialize the environment variables
PROC_NET_PSCHED and/or PROC_ROOT prior to the first libnl call, giving a
program more flexibility about where libnl should look.
3) Trivially faster startup time (although unlikely to be significant).
4) Compiler may be able to prove that the get_psched_settings() function
is unreachable and optimize appropriately, because the callers never
(directly or indirectly) use this method. This could occur, for
instance, in doing dead code elimination for programs which statically
link libnl.

Signed-off-by: Nick Kralevich <nnk@google.com>

https://github.com/thom311/libnl/pull/123

(cherry picked from commit 8e0ead4b5bcf22dcc210442629ebb416116ea1f0)

Test: Compiles and ueventd proc_net denial is gone.
Bug: 35197529
Change-Id: Iad9ea207315c92489617334edeba73053d67cf6b