uwsgi_opt_set_int: use strtol() and warn on non-numeric data

[awelzel]

[related to #2018 - when reading "After a lot of trouble shooting and digging through the uWSGI source" I thought maybe that can be more friendly]

This should not change the existing behavior of using 0 when
a non-numeric value is provided, but loudly logs:

$ uwsgi test.ini --processes 2x
[uWSGI] getting INI configuration from test.ini
[WARNING] non-numeric value "2x" for option "processes" - using 2 !
[WARNING] non-numeric value "true" for option "http-keepalive" - using 0 !
*** Starting uWSGI 2.1-dev-ade7d170 (64bit) on [Sun Jun 23 18:36:29 2019] ***
compiled with version: 8.3.0 on 23 June 2019 16:22:05
...

It might be saner to not even start with invalid options. However,
hypothetical setups with http-keepalive=off or http-keepalive=false may
then fail to start at all and people might get unahppy.

[awelzel]

Related #1245 and #1637

[xrmx]

@awelzel could you please run make tests with your commit on top of #2035?

UPDATE: just rebase to master and travis will run them

[awelzel]

@xrmx - ha! thanks for writing the tests. I hadn't looked around - next time I'll add some, too.

It's rebased:

Running suite(s): uwsgi strings                                 
 uwsgi opt parsing
[WARNING] non-numeric value "true" for option "" - using 0 !
[WARNING] non-numeric value "false" for option "" - using 0 !         
100%: Checks: 2, Failures: 0, Errors: 0          

[awelzel]

@xrmx - actually added a test for uwsgi_opt_set_int("", NULL, &result); which should correspond to optional_argument usage.

[xrmx]

Merged, thanks!