Shell variable names cannot be safely quoted with shell_escape() —
only alphanumeric characters are valid. Add validation that set-name
only matches [a-zA-Z0-9]+; invalid values are rejected with a
warning and the rename is skipped entirely.

The hostname value was written inside double quotes in
/etc/rc.conf.d/hostname. POSIX shell performs command substitution
inside double quotes, so a hostname containing $() or backticks would
be executed when the file is sourced (e.g., by rc(8)).

Switch to using the existing shell_escape() helper, which wraps values
in single quotes. In POSIX shell, single-quoted strings are completely
literal — no expansion or substitution of any kind is performed.

While the hostname is already validated to contain only
[a-zA-Z0-9.-], this change provides defense-in-depth so the output
format is safe regardless of future validation changes.

Reported by:	Yazdan Soltani <yazdan.soltani@gmail.com>

Currently, 'write_files' does not create parent directories, and
'runcmd' cannot be used here, since those scripts run after the files
have been written. The only workaround is to create the files in an
existing directory, such as '/root' or '/tmp', and then move those
files using 'runcmd', but this is cumbersome when there are many files,
even if they are small.

With this change, nuageinit now creates the parent directories for each
file using the path field, which mimics the same behavior as in
cloud-init.

Permissions and ownership can also be configured using 'runcmd'.

Reviewed by:            bapt@
Approved by:            bapt@
Differential Revision:  https://reviews.freebsd.org/D57395

Suggested by:	kevans

Posts instance data (hostname, instance_id, public keys) to a URL
using fetch(1). Supports:
- url: target URL
- post: list of data items to send, or 'all'
- tries: number of retry attempts (default 1)