N-of-N (XOR) seed splitting: user-level support

This patch introduces the commands `mmgen-seedsplit` and `mmgen-seedjoin`.
The first creates shares one at a time, while the second joins them to
recover the original seed.  By default, the default wallet is operated upon.

Shares are ordinary MMGen wallets and as such may be saved in any MMGen wallet
format, with one minor limitation: only one share in a given split may be in
hidden incognito format, and it must be the master share in the case of a
master share split.

For usage information and examples, see:

    mmgen-seedsplit --help
    mmgen-seedjoin --help

Relevant tests:

    test/test.py -e seedsplit
    ls -lrt test/tmp23/* # list the created files

    test/objtest.py SeedSplitSpecifier

Related commits:

    7311f474 - seed splitting: seed-level infrastructure
    237567bc - master shares

test/objtest_py_d/ot_btc_mainnet.py

@@ -13,6 +13,8 @@ from mmgen.obj import *
 from mmgen.seed import *
 from .ot_common import *
 
+ssm = str(SeedShareCount.max_val)
+
 tests = OrderedDict([
 	('AddrIdx', {
 		'bad':  ('s',1.1,10000000,-1,0),
@@ -202,4 +204,10 @@ tests = OrderedDict([
 			{'s':'P','ret':'P'},
 		)
 	}),
+	('SeedSplitSpecifier', {
+		'bad': ('M','αβ:2',1,'0:1','1:1','2:1','3:2','1:2000','abc:0:2'),
+		'good': (
+			('1:2','2:2','alice:2:2','αβ:2:2','1:'+ssm,ssm+':'+ssm)
+		)
+	}),
 ])