mount: unmount on sigint/sigterm, add --options and --mode, improve UI

[tstromberg]

A refactoring of the mount command to make it suck less, inspired by @jlaswell on Slack:

• If Ctrl-C or SIGTERM is received, unmount the target volume
• Add --options flag to allow arbitrary mount options to be passed
• Add --mode flag to set the file mode of the target mount
• Improve mount UI presentation and help strings

Example command:

minikube mount --mode 0777 --9p-version 9p2000 --uid 72 --gid 82 --options cache =fscache,nodevmap --msize=8192 /tmp/700:/tmp/700

Example output:

📁  Mounting host path /tmp/700 into VM as /tmp/700 ...
💾  Mount options:
    ▪ Type:     9p
    ▪ UID:      72
    ▪ GID:      82
    ▪ Version:  9p2000
    ▪ MSize:    8192
    ▪ Mode:     777 (-rwxrwxrwx)
    ▪ Options:  map[cache:fscache nodevmap:]
🚀  Userspace file server: ufs starting
✅  Successfully mounted /tmp/700 to /tmp/700

📌  NOTE: This process must stay alive for the mount to be accessible ...
^C🔥  Unmounting /tmp/700 ...
💣  Exiting due to interrupt signal

Closes #3853

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[tejal29]

just a small nit