Add MNE:<low>[:high][:cmd] @-code for mnemonic string attribute control

The "default" mnemonic string attributes are still set in ctrl/attr.cfg,
but now, each mnemonic string (e.g. from text.dat or passed to JS
console.mnemonics()) can include a "MNE:" @-code to over-ride the default
mnemonic string attributes (low, high, and command). If the "high"
attribute isn't specified, it defaults to the same color as the "low"
attribute with the "high intensity" flag flipped. The "cmd" attribute
defaults to whatever was included in the attr.cfg, if not specified.
Technically, the separator between the attributes can be any non-valid
attribute character (e.g. symbol).

Renamed (really old function) attrstr() to strtoattr() since its usage now
more closely resembles other std C strto* functions (has an 'endptr' arg).