Show correct value of \escapechar

[blefloch]

Currently, when reading \escapechar=-1, unravel shows \escapechar=92 instead of -1, but does the right thing. Namely,

\unravel{\escapechar=-1\message{\the\escapechar}}

gives

|> \escapechar =-1\message {\the \escapechar }
[===== Step 1 =====] \escapechar 
|| \escapechar 
|> =-1\message {\the \escapechar }
[===== Step 2 =====] =
|| \escapechar =
|> -1\message {\the \escapechar }
[===== Step 3 =====] -
|| \escapechar =-
|> 1\message {\the \escapechar }
[===== Step 4 =====] 1
|| \escapechar =-1
|> \message {\the \escapechar }
[===== Step 5 =====] Set \escapechar(\escapechar)=92
|| 
|> \message {\the \escapechar }
[===== Step 6 =====] \message = \message
|| \message 
|> {\the \escapechar }
[===== Step 7 =====] \message = \message
|| \message 
|| {
|| \the 
|> \escapechar }
[===== Step 8 =====] \the =>-1
|| \message 
|| {-1
|> }
-1
[===== Step 9 =====] \message {-1}
|| 
|> 
[===== The end! =====]