Define LCME_FL_IMMEDIATE and add userspace layout support.

Flag definition (lustre_user.h):

• Add to enum lov_comp_md_entry_flags
• Add to LCME_KNOWN_FLAGS, LCME_USER_COMP_FLAGS, LCME_CL_COMP_FLAGS, LCME_TEMPLATE_FLAGS

Display and CLI (lustreapi.h):

• Add { LCME_FL_IMMEDIATE, "immediate" }

  to comp_flags_table so --comp-flags immediate works

Validation (liblustreapi_layout.c):

• LCME_FL_IMMEDIATE and LCME_FL_PARITY are mutually exclusive on a component

Reference EC patches:

• Gerrit 61962: LU-12187 llapi: EC layout creation support (pattern for flag + llapi)

Prototype patches:

• Gerrit 59597: immediate mirror flag definition
• Gerrit 59570: userspace tools for immediate flag