Add mallocinfo mode to getmemoryinfo RPC

Summary:
This adds a mode argument to getmemoryinfo. By default the output
will remain the same. However if a mode argument of mallocinfo is
provided the result of glibc malloc_info (if available) will
be returned as a string, as-is.

This is useful for tracking heap usage over time or troubleshooting
memory fragmentation issues.

This is a backport of Core PR10038

Test Plan:

make check
ninja all check

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1899