Add the -b / --batch-grouping flag and the {a,b,c} brace syntax to
pmcstat(8). For brace expressions in -p / -P / -s / -S the tool now:

1. creates a kernel group via pmc_group_create();
2. for each event token allocates a deferred pmcid with pmc_allocate_group() and registers it via pmc_group_add(), marking the first one as the leader;
3. commits the group with pmc_group_commit() before any pmc_attach.

A brace containing a single event falls back to the legacy
pmc_allocate path so that "{a}" stays equivalent to "a".

The man page documents the syntax, the multiplex semantics enabled by
PMC_F_GROUP_MUX, and the all-or-none commit guarantee.

Sponsored by: AMD
Signed-off-by: Raghavendra K T <raghavendra.kt@amd.com>