libbpf-tools: add mountsnoop

libbpf-tools: add mountsnoop

This commit adds a new libbpf tool mountsnoop.
It has the same functionalities just as its
counterpart in BCC tools. The default output
is the same.

```
$ mountsnoop
COMM             PID     TID     MNT_NS      CALL
dockerd          1827    1903    4026531840  mount("overlay", "/data/docker/overlay2/153e6b58322c64cf4b2aac1b9caba42d390481a7d33a2bffe0eb858943d49fb6-init/merged", "overlay", 0x0, "index=off,lowerdir=/data/docker/overlay2/l/GWTHHZ2C3PYGAJ5GLTWLHMHHKR,upperdir=/data/docker/overlay2/153e6b58322c64cf4b2aac1b9caba42d390481a7d33a2bffe0eb858943d49fb6-init/diff,workdir=/data/docker/overlay2/153e6b58322c64cf4b2aac1b9caba42d390481a7d33a2bffe0eb858943d49fb6-init/work") = 0
dockerd          1827    1903    4026531840  umount("/data/docker/overlay2/153e6b58322c64cf4b2aac1b9caba42d390481a7d33a2bffe0eb858943d49fb6-init/merged", MS_NOSUID) = 0
```

Also, we provide a detailed mode enabled by -d
option which displays each mount/umount syscall
vertically with more field. In this way, the
output looks more friendly.

```
$ mountsnoop -d -t
PID:    1827
TID:    1864
COMM:   dockerd
OP:     MOUNT
RET:    0
LAT:    246us
MNT_NS: 4026531840
FS:     overlay
SOURCE: overlay
TARGET: /data/docker/overlay2/5fc51d4e4820082177751a8aadf3f42a751c86aff1e0efbc1a5e6af345ee205a-init/merged
DATA:   index=off,lowerdir=/data/docker/overlay2/l/GWTHHZ2C3PYGAJ5GLTWLHMHHKR,upperdir=/data/docker/overlay2/5fc51d4e4820082177751a8aadf3f42a751c86aff1e0efbc1a5e6af345ee205a-init/diff,workdir=/data/docker/overlay2/5fc51d4e4820082177751a8aadf3f42a751c86aff1e0efbc1a5e6af345ee205a-init/work
FLAGS:  0x0

PID:    1827
TID:    1864
COMM:   dockerd
OP:     UMOUNT
RET:    0
LAT:    95us
MNT_NS: 4026531840
FS:
SOURCE:
TARGET: /data/docker/overlay2/5fc51d4e4820082177751a8aadf3f42a751c86aff1e0efbc1a5e6af345ee205a-init/merged
DATA:
FLAGS:  MS_NOSUID
```

Signed-off-by: Hengqi Chen <chenhengqi@outlook.com>