MINOR: check: do not increment global maxsock at runtime

MINOR: check: do not increment global maxsock at runtime

global maxsock is used to estimate a number of fd to reserve for
internal use, such as checks. It is incremented at startup with the info
from the config file.

Disable this incrementation in checks functions at runtime. First, it
currently serves no purpose to increment it after startup. Worse, it may
lead to out-of-bound accesse on the fdtab.

This will be useful to initiate checks for dynamic servers.