DEBUG: cli: add a new "debug dev fd" expert command

DEBUG: cli: add a new "debug dev fd" expert command

This command will scan the whole file descriptors space to look for
existing FDs that are unknown to haproxy's fdtab, and will try to dump
a maximum number of information about them (including type, mode, device,
size, uid/gid, cloexec, O_* flags, socket types and addresses when
relevant). The goal is to help detecting inherited FDs from parent
processes as well as potential leaks.

Some of those listed are actually known but handled so deep into some
systems that they're not in the fdtab (such as epoll FDs or inter-
thread pipes). This might be refined in the future so that these ones
become known and do not appear.

Example of output:

 $ socat - /tmp/sock1 <<< "expert-mode on;debug dev fd"

    0 type=tty. mod=0620 dev=0x8803 siz=0 uid=1000 gid=5 fs=0x16 ino=0x6 getfd=+0 getfl=O_RDONLY,O_APPEND
    1 type=tty. mod=0620 dev=0x8803 siz=0 uid=1000 gid=5 fs=0x16 ino=0x6 getfd=+0 getfl=O_RDONLY,O_APPEND
    2 type=tty. mod=0620 dev=0x8803 siz=0 uid=1000 gid=5 fs=0x16 ino=0x6 getfd=+0 getfl=O_RDONLY,O_APPEND
    3 type=pipe mod=0600 dev=0 siz=0 uid=1000 gid=100 fs=0xc ino=0x18112348 getfd=+0
    4 type=epol mod=0600 dev=0 siz=0 uid=0 gid=0 fs=0xd ino=0x3674 getfd=+0 getfl=O_RDONLY
   33 type=pipe mod=0600 dev=0 siz=0 uid=1000 gid=100 fs=0xc ino=0x24af8251 getfd=+0 getfl=O_RDONLY
   34 type=epol mod=0600 dev=0 siz=0 uid=0 gid=0 fs=0xd ino=0x3674 getfd=+0 getfl=O_RDONLY
   36 type=pipe mod=0600 dev=0 siz=0 uid=1000 gid=100 fs=0xc ino=0x24af8d1b getfd=+0 getfl=O_RDONLY
   37 type=epol mod=0600 dev=0 siz=0 uid=0 gid=0 fs=0xd ino=0x3674 getfd=+0 getfl=O_RDONLY
   39 type=pipe mod=0600 dev=0 siz=0 uid=1000 gid=100 fs=0xc ino=0x24afa04f getfd=+0 getfl=O_RDONLY
   41 type=pipe mod=0600 dev=0 siz=0 uid=1000 gid=100 fs=0xc ino=0x24af8252 getfd=+0 getfl=O_RDONLY
   42 type=epol mod=0600 dev=0 siz=0 uid=0 gid=0 fs=0xd ino=0x3674 getfd=+0 getfl=O_RDONLY