[roll] Roll fuchsia [debugger] Better handle runtime symbol updates.

Automatically resymbolize in various cases to catch updates to the
symbol search path or the files on disk that the user may have made:

 • Automatically try to reload symbols for unsymbolized libraries every
   time the library list is requested (the "libs" command). This means
   that the if the user updates their symbol search path and types
   "libs", most missing symbol information will be resolved.

 • Automatically refresh stack frames for all stopped threads when
   symbols have changed. This will automatically symbolize all stacks
   when the above "libs" command detects a change.

Provide ways for the user to force updates for cases where they suspect
something might be stale:

 • Add an option to force-update the library and symbol list
   ("libs --reload"). This allows the user to do the reload if something
   has been manually changed or they suspect the debugger is not
   updating automatically for some reason.

 • Add an option to force-update the stack frame list and symbols
   ("frame --force" and "backtrace --force"). This allows the user to
   request both resyncing the thread's current frames and the associated
   symbol information if they suspect something has changed out from
   under them.

Clarify symbol index dumping. Some users have been confused why
"sym-stat --dump-index" contains nothing. Add a reference to this
command from the help for the settings that feed into this data.

Original-Bug: 117110

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/783482
Original-Revision: a12f40c95d46b4f6e6cd392de25484bc87f31195
GitOrigin-RevId: de3498430f201f8167fa6de450dd7b70295ef411
Change-Id: If442b74043f11cd1239826f65abf44ad349bd05c