tests/hgterm.ti

config: explicitly track the use of the standard default value

We introduce a small object used to detect that no specific default value has
been passed to 'ui.config'. We need this explicit special value since "None" is
a valid and common default value.

The end goal here is to make progress on a centralised and explicit declaration
of the available config option. A first good usecase for this are "default"
value.  Before starting looking further down this alley we needs to rework the
handling of default value in the 'ui' object to have all configxyz methods going
through the same logic. This is the first changeset on this trek.

hgterm,
	am, km, mir, msgr, xenl,
	colors#8, cols#80, it#8, lines#24, pairs#64,
	acsc=``aaffggiijjkkllmmnnooppqqrrssttuuvvwwxxyyzz{{||}}~~,
	bel=^G, bold=\E[1m, clear=\E[H\E[2J, cr=\r,
	csr=\E[%i%p1%d;%p2%dr, cub=\E[%p1%dD, cub1=\b,
	cud=\E[%p1%dB, cud1=\n, cuf=\E[%p1%dC, cuf1=\E[C,
	cup=\E[%i%p1%d;%p2%dH, cuu=\E[%p1%dA, cuu1=\E[A,
	dch=\E[%p1%dP, dch1=\E[P, dl=\E[%p1%dM, dl1=\E[M,
	ed=\E[J, el=\E[K, enacs=\E)0, home=\E[H, ht=\t,
	hts=\EH, il=\E[%p1%dL, il1=\E[L, ind=\n,
	is2=\E[m\E[?7h\E[4l\E>\E7\E[r\E[?1;3;4;6l\E8, kbs=\b,
	kcub1=\EOD, kcud1=\EOB, kcuf1=\EOC, kcuu1=\EOA,
	kdch1=\E[3~, kf1=\E[11~, kf10=\E[21~, kf11=\E[23~,
	kf12=\E[24~, kf13=\E[25~, kf14=\E[26~, kf15=\E[28~,
	kf16=\E[29~, kf17=\E[31~, kf18=\E[32~, kf19=\E[33~,
	kf2=\E[12~, kf20=\E[34~, kf3=\E[13~, kf4=\E[14~,
	kf5=\E[15~, kf6=\E[17~, kf7=\E[18~, kf8=\E[19~,
	kf9=\E[20~, kfnd=\E[1~, kich1=\E[2~, kmous=\E[M,
	knp=\E[6~, kpp=\E[5~, kslt=\E[4~, op=\E[m, rc=\E8,
	rev=\E[7m, ri=\EM, rmacs=^O, rmcup=\E[2J\E[?47l\E8,
	rmir=\E[4l, rmkx=\E[?1l\E>, rmso=\E[m, rmul=\E[m,
	rs2=\E[m\E[?7h\E[4l\E>\E7\E[r\E[?1;3;4;6l\E8, sc=\E7,
	setab=\E[4%p1%dm, setaf=\E[3%p1%dm, sgr0=\E[m,
	smacs=^N, smcup=\E7\E[?47h, smir=\E[4h,
	smkx=\E[?1h\E=, smso=\E[7m, smul=\E[4m, tbc=\E[3g,
	u6=\E[%i%d;%dR, u7=\E[6n, u8=\E[?1;2c, u9=\E[c,