remove 'register' usage

I don't think we need to add register modifier to variables right now for this
piece of code. Otherwise it has following compile warnings:

/Users/lgq/sysdig/userspace/libsinsp/filterchecks.cpp:1044:2: warning: 'register' storage class specifier is deprecated
      [-Wdeprecated-register]
        register int dt, dir;
        ^~~~~~~~~
/Users/lgq/sysdig/userspace/libsinsp/filterchecks.cpp:1044:2: warning: 'register' storage class specifier is deprecated
      [-Wdeprecated-register]
        register int dt, dir;
        ^~~~~~~~~
/Users/lgq/sysdig/userspace/libsinsp/filterchecks.cpp:1045:2: warning: 'register' storage class specifier is deprecated
      [-Wdeprecated-register]
        register struct tm *gmt, *loc;
        ^~~~~~~~~
/Users/lgq/sysdig/userspace/libsinsp/filterchecks.cpp:1045:2: warning: 'register' storage class specifier is deprecated
      [-Wdeprecated-register]
        register struct tm *gmt, *loc;
        ^~~~~~~~~
correct the pos's size and then fix the following two compile warnings:

/Users/lgq/github/sysdig/userspace/libsinsp/filterchecks.cpp:209:11: warning: comparison of constant 18446744073709551615
      with expression of type 'uint32_t' (aka 'unsigned int') is always true [-Wtautological-constant-out-of-range-compare]
                        if(pos != string::npos)
                           ~~~ ^  ~~~~~~~~~~~~
/Users/lgq/github/sysdig/userspace/libsinsp/filterchecks.cpp:315:11: warning: comparison of constant 18446744073709551615
      with expression of type 'uint32_t' (aka 'unsigned int') is always true [-Wtautological-constant-out-of-range-compare]
                        if(pos != string::npos)
                           ~~~ ^  ~~~~~~~~~~~~
Add Travis continous integration support

Since we can't rely on a proper kernel configuration on Travis build
machines (kernel headers, CONFIG_TRACEPOINTS, etc.), building the driver
is currently disabled.

Travis build status will be available at [1].

[1] - https://travis-ci.org/draios/sysdig

Signed-off-by: Christian Babeux <christian.babeux@0x80.ca>
Add Travis CI build status to README

Signed-off-by: Christian Babeux <christian.babeux@0x80.ca>
Fix: get_buf_pointers() declaration should be static on non-windows platform

When compiling sysdig with Clang, one might run into this error:

Linking CXX executable sysdig
../libscap/libscap.a(scap.c.o): In function `scap_readbuf':
/home/travis/build/cbab/sysdig/userspace/libscap/scap.c:488: undefined reference to `get_buf_pointers'
/home/travis/build/cbab/sysdig/userspace/libscap/scap.c:506: undefined reference to `get_buf_pointers'
../libscap/libscap.a(scap.c.o): In function `check_scap_next_wait':
/home/travis/build/cbab/sysdig/userspace/libscap/scap.c:565: undefined reference to `get_buf_pointers'
collect2: ld returned 1 exit status
make[2]: *** [userspace/sysdig/sysdig] Error 1
make[1]: *** [userspace/sysdig/CMakeFiles/sysdig.dir/all] Error 2
make: *** [all] Error 2

This can be explained by the semantic difference of the "inline" keyword
with GCC compilers vs. Clang [1]. The fix is to add the static keyword,
since this function is only used in this compile unit.

[1] - http://clang.llvm.org/compatibility.html#inline

Signed-off-by: Christian Babeux <christian.babeux@0x80.ca>