mkimg: Add ms-basic-data alias for GPT

While preparing GPT-schemed RaspberryPi images for the NanoBSD
Reimagined GSoC 2026 project, a discrepancy was identified between
mkimg(1) and gpart(8) regarding Microsoft Basic Data partitions (GUID
!ebd0a0a2-b9e5-4433-87c0-68b6b72699c7).

Currently, mkimg(1) relies on the MBR-centric name "ntfs" to identify
this partition type under the GPT scheme. Conversely, gpart(8)
identifies this type as "ms-basic-data".

To allow automation scripts (such as those consuming from gpart backup)
to use a common partition type across tools, add ALIAS_MS_BASIC_DATA as
a valid alias.

This is part of a larger effort to avoid a custom, MBR-based image
generation logic for embedded SoCs like the Raspberry Pi, standardizing
on GPT layouts across all supported FreeBSD embedded devices.

Reviewed by: imp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D58198

(cherry picked from commit 67f1c082b5ec37e2060b74e6f1f952ed38761468)