[pxzomajj] GMP ECM autoincrement

Analyzing table of optimal B1 in GMP-ECM's README file. The sequence is neither linear nor geometric. It is roughly quadratic. b1_ainc.c (as of 6.3-5ubuntu1) needs updating.

Min	N	Max	Linear	Geometric	Log log
11e3	74	5e4	527.027027	0.142856	2.456124
5e4	214	25e4	934.579439	0.0570377	2.274716
25e4	430	1e6	1744.186047	0.0314601	2.230921
1e6	904	3e6	2212.389381	0.0160494	2.131485
3e6	2350	11e6	3404.255319	0.00676381	2.047746
11e6	4480	43e6	7142.857143	0.00385742	2.055486
43e6	7553	11e7	8870.647425	0.00238583	2.018008
11e7	17769	26e7	8441.668074	0.00105949	1.923939
26e7	42017	85e7	14041.935407	0.000480654	1.897046

$d=$b-$a; $s=sprintf "<tr><td>%s %d %s %f %g %f</td></tr>", $a, $n, $b, $d/$n, log($d)/$n, log($d)/log($n); $s=~s, ,</td><td>,g